1. Field of the Invention
This invention relates to an irreversible compression apparatus and to a method and program for controlling the operation of this apparatus.
2. Description of the Related Art
Irreversible compression allows some dropout of data but features a high compression efficiency. Even when decoded, data that has been compressed irreversibly cannot be restored completely to the data that prevailed prior to compression. If a desired image quality is desired with regard to an irreversibly compressed image, processing is executed as follows: For example, image data representing an original image is irreversibly compressed, the irreversibly compressed image data is decoded, and irreversible compression, decoding and calculation of an image degradation index of the decoded image data are repeated until the image degradation index represents the desired image.
In a transcoding proxy that acts as a relay between a plurality of client devices and a plurality of servers connected via a communication network, there is a technique that enhances the advantage of transcoding, namely how to send a high-quality image to each client device within transmission-delay tolerance (Patent Document 1). First, the network characteristics are analyzed, an estimated value of image transmission time is acquired and a decision is rendered as to whether or not to apply transcoding. If transcoding is carried out, then transcoding parameters for which output image size and a tradeoff between quality and size will be optimized are predicted statistically and adjusted. A tradeoff relating to selection of transcoding parameters can also be converted dynamically by the user.
Further, in order to maintain a prescribed image quality for a decoded image and to realize as high a compression ratio as possible, there is a technique for predicting image quality by measuring such physical quantities as the input image and states of the input and output devices, and controlling image quality using coding parameters that have been adapted to these physical quantities (Patent Document 2).
Furthermore, there is a technique, which is used in an imaging apparatus such as a facsimile machine, for calculating an image degradation index (SSIM: Structural SIMilarity) value in comparatively accurate fashion (Patent Document 3).
Further, there is a coding apparatus for setting and coding a group of coding parameters suitable for a plurality of partial images, (Patent Document 4), a coding apparatus having excellent operability and capable of being fabricated at low cost (Patent Document 5), and a technique for obtaining decoded images of stable quality (Patent Document 6).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-222332
[Patent Document 2] Japanese Patent Application Laid-Open No. 10-75369
[Patent Document 3] Japanese Patent Application Laid-Open No. 2010-206244
[Patent Document 4] Japanese Patent Application Laid-Open No. 2006-60343
[Patent Document 5] Japanese Patent Application Laid-Open No. 2000-175048
[Patent Document 6] Japanese Patent Application Laid-Open No. 5-176173
Since irreversible compression, decoding and calculation of the image degradation index often are time consuming, it often takes time to obtain the desired image quality. With the technique described in Patent Document 1, it is difficult to obtain an output image that falls within the desired image quality. Since a subjective image degradation index is utilized in the technique described in Patent Document 2, objectivity is lacking. In addition, since the relationship between image quality and the degradation parameters is not updated, image quality cannot be controlled efficiently. With the technique described in Patent Document 3, a decline in image quality due to compression is not measured from the first. Further, obtaining the desired image quality takes too much time with the techniques described in Patent Documents 4 to 6.